1. Field of the Invention
The invention relates to the field of temperature measurement and control, in particular in an induction furnace for heating the end of a zirconium alloy nuclear fuel rod in the presence of an oxidizing gas, to form a protective zirconium oxide layer in an area of the fuel rod at which a fuel assembly structure tends to fret captured debris against the fuel rod. According to the invention, a probe that is configured to correspond to a fuel rod is provided with an array of spaced thermocouples for monitoring the temperature contour obtained in the induction furnace, enabling precise measurement and control of the temperature profile that occurs when a fuel rod is treated in the induction furnace under the same conditions.
2. Prior Art
In a nuclear power plant, coolant is heated in a vessel disposed along a primary coolant circuit, by nuclear fuel in the form of vertically elongated fuel rods carried in fuel assemblies. Each fuel rod comprises a stack of enriched uranium pellets in a hollow tube typically made of a zirconium alloy such as Zircalloy. The zirconium alloy presents a low nuclear cross section to neutrons that carry on the fission reaction leading to heating of the fuel. Zirconium is a highly active metal that, like aluminum, seems passive because a stable and cohesive thin zirconium oxide (ZrO.sub.2) film forms on the surface in the presence of oxygen, e.g., when exposed to air or water. Formation of the oxide layer is accelerated with heating.
Zirconium is advantageous for its subatomic properties, but is not the most durable of metals. It is possible to treat zirconium fuel rod tubes prior to their use in a reactor, to add a protective cladding layer that is more durable, thicker and/or more chemically resistant than the basic tube material. Such a cladding can comprise a plated-on alloy or a distinct metal. Heating of the fuel rod tube may be involved in the process. Typically the entire tube is treated; however, it is also possible to treat only particular areas that are considered vulnerable to corrosion or the like, for example the inside of the tube. Particular claddings are chosen for corrosion resistance, especially resistance to corrosion from reaction of the zirconium with iodine and other elements released during fission of the nuclear fuel. Examples of protective treatment of the inside or outside of fuel rod tubes are disclosed, for example, in the following patents:
______________________________________ 4,100,020 Andrews 4,233,086 Vesterlund 4,411,861 Steinberg 4,609,524 Ferrari 4,613,479 Foster 4,659,545 Ferrari 4,675,153 Boyle et al. 4,894,203 Adamson 5,026,516 Taylor 5,073,336 Taylor 5,137,683 Hertz et al. ______________________________________
In pressurized water and boiling water reactors, the fuel rods are grouped in fuel assembly structures that have vertically spaced grids for holding the fuel rods in a parallel array, whereby a number of the fuel rods can be handled as a fuel assembly unit. The liquid coolant is heated in a vessel by fission in the fuel rods, causing a vigorous and turbulent upward flow of coolant over the fuel rods due to convection. The fuel assembly grids are openwork panels disposed perpendicular to the elongation of the fuel rods, with spring structures that bear against the fuel rods to hold them in place. However, in the turbulent flow of the reactor coolant, loose metallic debris may be stopped by the grids, especially at the endmost grid facing the direction of flow. This captured debris vibrates against the fuel rods, leading to fretting damage to the zirconium alloy tube in the area of the endmost grid. A breach of the fuel rod tube can lead to release of fuel into the coolant, which is undesirable due to the resulting circulation of radioactive material with the coolant.
Fretting damage to the fuel rods has been found to occur primarily during the first cycle of their irradiation. Fissile heating of the fuel rods in the water coolant of the reactor during use thickens the zirconium oxide layer on the outer surfaces of the fuel rods and thereafter protects the fuel rods from fretting damage. Operational temperatures and pressures in a pressurized water reactor, for example, may be on the order of 300.degree. to 400.degree. C. and 150 bar. As described in commonly owned U.S. patent application Ser. No. 08/025,361, filed Mar. 2, 1993, it is possible to pretreat the fuel rod tubes to form a protective zirconium oxide layer prior to installation of the fuel rods. The protective layer is formed along the endmost four to eight inches (10 to 20 cm) of the fuel rod and reduces or eliminates fuel rod failure during initial use. This end portion of the fuel rod is the approximate length that protrudes from the lowermost grid of the fuel assembly, in the direction facing the coolant flow, i.e., the area in which the lowermost grid is likely to capture debris that will fret against the fuel rod.
The protective zirconium oxide layer can be formed by heating the fuel rod tube in the presence of oxygen. The thickness of the resulting ZrO.sub.2 layer is a function of the time of heat treatment, the temperature, and the oxygen concentration in the treating atmosphere, typically air. It is desirable to form a uniform coating that completely encompasses the end of the tube, and has a depth of two to fifteen microns. Formation of a protective oxide by heating in this manner is of course much easier than application of an alloy cladding, for example requiring a plasma arc or other process, such as in U.S. Pat. No. 5,227,129 - Bryan et al.
There are a variety of means by which a fuel rod tube can be heated, for example using convection, laser irradiation, application of a flame, etc. An advantageous method is heating via electrical induction. The end of the fuel rod tube is placed in an electrical induction furnace having coils coupled to an AC power source, for inducing a current in the metallic zirconium material. Induced eddy currents dissipate electrical power by resistance heating. This form of heating is advantageous in that the power can be concentrated at the area to be treated (at least subject to conduction of the heat to the remainder of the fuel rod tube).
Preferably, the induction furnace is only barely larger than the fuel rod or rods being treated. In this manner, the electromagnetic field intensity can be maximized by minimizing the gap between the coils (or the ferromagnetic material coupling the field to the fuel rod) in the magnetic circuit. To achieve uniform application of electromagnetic energy, a series of adjacent or spaced coil pairs can be disposed on opposite sides of the fuel rod tube, each of the coils in a pair being energized at opposite polarity and each coil pair encompassing a limited axial length of the fuel rod. These individual coil pairs can be controlled separately, for applying the precise power level needed to achieve uniform heating of the fuel rod tube.
It is advantageous to apply the minimum power necessary to obtain the required thickness of ZrO.sub.2 over all the end of the fuel rod tube to be treated. A uniform coating of ZrO.sub.2 that is of sufficient thickness over all the area of treatment requires precise temperature control. To achieve the uniform heating needed over the length of the fuel rod end, some means is needed to measure the induced heat from the respective coils, and to adjust or control the power level applied to the coils as necessary. It is not entirely adequate simply to apply equal power levels to each of the coil pairs, because the effects of the induction heating may be uneven even if the field strengths are equal, due to the variations in the eddy currents induced in the fuel rods occurring due to differences in geometry along the fuel rods. For example, the induced currents may vary between the center of the treated length of the fuel rod and the extreme end of the fuel rod, or between the center and the area at which the treated portion meets the proximal portion of the fuel rod, due to end effects and due to the adjacent conductive metal, respectively. The present invention concerns a method and apparatus for sensing the temperature effects of electromagnetic induction in a fuel rod by providing a temperature probe structured to simulate the conductive and/or resistive aspects of the fuel rod tube, and has an array of thermocouples disposed to measure the temperatures at specific spaced points. In this manner a temperature profile can be measured and used to adjust the electromagnetic field strengths of the coil pairs, for obtaining a desired temperature profile when the probe is replaced by an actual fuel rod to be treated.
Temperature probes for measuring the heat in a furnace are known generally. In a typical application, the probe is moved to different areas of the furnace in order to develop a temperature profile from a series of successive measurements. It is also possible to use an array of temperature sensors on a probe, for example as shown by U.S. Pat. Nos. 4,176,554 or 4,242,907, both to Kazmierowicz, or 4,098,122- Landman et al. For such uses, the spaced temperature sensing elements of the probe are intended to measure the ambient temperature at different points in the furnace or kiln. It is assumed that the ambient temperature as so measured is the temperature to which the workpieces will be heated when at the corresponding location. The furnace or kiln is then adjusted to obtain a desired temperature profile.
This technique cannot be used effectively if the presence or movement of the workpiece being heated in the furnace has an effect on the temperature to which the workpiece is heated. In a tunnel furnace, for example, cool workpieces entering the furnace reduce the ambient temperature there, other things being equal. Similarly, where a tunnel furnace has workpieces moving from a zone at one temperature into a zone at another temperature, it will take a certain time for the workpieces to reach the temperature of the new zone, assuming that the workpieces remain long enough to reach the ambient temperature at all. The reason for providing a probe having spaced temperature sensors is to obtain a measurement of the differences in temperature from point to point in the furnace, for example due to the workpieces, so that such effects can be addressed.
Temperature probes as disclosed in Kazmierowicz or Landman et al. are not suitable for measuring the temperature profile of an electromagnetic induction furnace, for adjustment of the field generating means as needed to develop the required temperature profile in workpieces when inserted to be heated. The electrical induction form of heating apparatus is particularly affected by the presence of the workpiece, because for the most part the heat is generated in the workpiece rather than in the ambient air of the furnace. Although indirect temperature sensing means could be installed in the furnace to sense the workpiece temperature at different points, this is a complex and expensive solution to the problem. According to the present invention, a probe having spaced temperature sensors is configured to resemble a workpiece, in particular the end of a fuel rod tube to be heated for producing a protective oxide layer, whereby currents induced in the probe and the temperature to which the probe is heated, closely model the situation for actual fuel rod workpieces.